"Net Result": Using Plankton Tow Material to Evaluate Geochemical Variability between Individual Foraminifera on a Single Day in the Ocean

Geochemical analyses of planktic foraminifera shells provide an archive of past environments, with average environmental conditions reconstructed from a bulk measurement of many shells combined for a single measurement. Recently, individual foraminiferal analysis (IFA) has grown in popularity, leveraging the range of geochemical proxies recorded by a suite of individual shells to characterize the environmental and oceanographic variability during select "time slices" in the paleo-record. Numerous studies have used plankton tow and sediment trap material to calibrate the relationship between oceanographic variables and pre-depositional shell chemistry. However, few studies have yet to explore the variability between individual foraminifera in a single contemporaneous population. Here we present paired geochemical measurements on individual shells collected in depth-stratified plankton tows near Green Island, Taiwan (22.667ºN, 121.483ºE). We measure shell trace element ratios using LA-ICP-MS depth profiling, and then measure stable isotope ratios (δ¹⁸O, δ¹³C) on the same individual shells. We use water sample geochemistry, measured environmental variables, and published proxy relationships to predict the range of values for stable isotope and trace element chemistry. We compare these predicted values to those obtained from the trace element and isotope analyses. This study demonstrates the range of geochemical values that can be captured by a suite of living foraminifera shells on a single day, and provides a baseline for IFA applications, which include reconstructions of paleo-monsoon intensity and the El Niño Southern Oscillation. This study also highlights the need for proxy calibrations specific to single-shell geochemical analyses in select regional oceanographic settings.